1. Field of the Invention
The present invention relates to a portable electronic apparatus such as a portable computer, which has a circuit board having a TCP (Tape Carrier Package) mounted on it, and more particularly to a structure dissipating heat from the
2. Description of the Related Art
Book-type and notebook-type portable computers recently developed are made increasingly compact and capable of performing more and more functions. Each portable computer comprises a circuit board and an package. The LSI package is mounted on the circuit board. It has an increased data-storage capacity and can perform many functions. Inevitably it has a chip size and many pins. The larger the LSI package, the greater area it occupies on the circuit board, decrease this area without reducing the number of pins, it would be necessary to minimize the lead pitch of the LSI package.
Known as LSI packages which are relatively small and yet has many pins are a PGA (Pin Grid Array) and a QFP (Quad flat Package). If these LSI packages have a large chip size and many pins, however, they will be no longer small enough to be incorporated into multi-function portable computers.
An LSI package which has many pins and which is compact nonetheless is now attracting attention. This is a TCP (Tape Carrier Package). A TCP comprises a resin film, a number of leads provided on the film, and an IC chip mounted on the film. The leads have their end portions extending from the edges of the resin film. They have their ends soldered to the pads which is provided on the circuit board.
The IC chip of the TCP is exposed outside a resin molding, not embedded within the molding. The TCP is therefore mechanically less strong than the PGA. Since the TCP is mechanically fragile, a heatsink having a number of heat-radiating fins can hardly be attached to the TCP to absorb a great amount of heat which the TCP generates while operating. It is therefore necessary to dissipate heat from the any TCP incorporated into a computer.
There are know some methods of dissipating heat from a TCP used in a computer. One of them is to provide an electric fan in the housing of the computer, for cooling the TCP. The use of the electric fan is disadvantageous in some respects. First, the housing needs to be large enough to accommodate it, inevitably making the computer less compact. Second, the electric fan makes much noise while operating. Third, the manufacturing cost of the computer increases. Fourth, the fan consumes much power, greatly increasing the power consumption of the computer, and the operating time of the computer will be short if the computer is a battery-driven one.
Another known method of dissipating heat from the TCP used in a computer is to adhere the IC chip to the circuit board, causing the heat to propagate to the circuit board from the TCP including the IC chip. This method cannot dissipate the heat effectively from the circuit board, because the circuit board has but a limited size. The heat from the TCP accumulate in the circuit board, heating the circuit board to a high temperature. As the circuit board is heated, the components mounted on the circuit board together with the TCP (e.g., LSI packages and circuit components) are inevitably heated. When excessively heated, the components may malfunction.
As regards technique of dissipating heat from an IC chip, Jpn. Pat. Appln. KOKAI Publication No. 5-52079 discloses an electronic device which has an IC chip and which is mounted on a circuit board. The circuit board has a through hole which is relatively large and whose inner surface is plated. A heat-radiating plate is provided on the lower surface of the circuit board, with a projection inserted in the through hole of the circuit board. The projection is made of good thermal conductivity, such as copper or brass. The projection contacts the electronic device. Hence, the electronic device generates while operating propagates into the heat-radiating plate through the projection. This technique of dissipating heat from an IC chip has the following drawbacks, however.
The projection of the heat-radiating plate contacts the layer plated on the inner surface of the through hole. The heat transmitted from the electronic device to the projection is therefore dissipated to the circuit board through the plated layer. The heat is readily transferred to the circuit board because the hole is much smaller than the device and also because the device is connected to the circuit board by a layer of solder or thermal compound. The heat affects the circuit board very much.
Furthermore, since the through hole is much smaller than the electronic device, the contacting area of the device and the projection is small. Heat cannot be transferred with high efficiency from the device to the projection. The electronic device cannot radiate heat sufficiently, though a heat-transmitting path extends from the device to the heat-radiating plate.